In the forest industry, logs are loaded for transport by means of a hydraulically operated grapple attached to the carrier arm of a log skidder, track mounted loader, processor or other vehicle. This grapple has two claws which are opened and closed by the use of a hydraulic piston-cylinder which provides the force necessary for lifting or transporting heavy loads. The grapple is attached to the end of the carrier arm by means of a universal linkage having two axes of rotation set 90 degrees to one another (henceforth called a twin axis universal joint), thereby allowing free rotation in all directions.
While maneuverability of the grapple is desirable to facilitate loading, a number of problems are also associated with the free movement of an empty grapple. Primarily, the uncontrolled movement of the grapple poses a safety hazard to workers and results in excessive and premature wear on the component parts of the carrier arm linkage. Thus, dampening of the grapple movement is necessary.
Previous devices designed to control grapple movement have utilized multiple disks composed of different materials and applied by various means (see U.S. Pat. Nos. 4,573,728 to Johnson, 4,717,191 to Farmer and 4,810,020 to Powell). These techniques were subject to a number of difficulties including premature wear, erratic performance, the production of heat through friction, and a susceptibility to contamination from oil and moisture from the external environment.
U.S. Pat. No. 4,572,567 to Johnson, describes a snubber containing two rings bearing tapered surfaces driven together by hydraulic pressure. The configuration of the component parts poses a number of potential problems. Firstly, a frictional wedging action between the tapered elements and the axial pin is relied upon in order to indirectly increase frictional drag between the tubular housing and the linkage housing, thereby resulting in dampening; this arrangement places unnecessary torque on the axial pin, which is fixed and held from rotating on the linkage housing only by a single external bolt. Because of its location, this bolt is susceptible to physical damage from the external environment. Its failure would result in the free rotation of the axial pin, and a consequent disabling of the dampening action.
Secondly, the amount of friction produced by snubbing a two inch pin with such small friction-bearing surfaces is not sufficient to stop or even slow down a moving grapple weighing one ton. These friction-bearing surfaces would be subject to extreme wear.
Thirdly, Johnson describes a pair of plastic lip-type seals which enclose the hydraulic cavity, forming seals with the surface of the bore and the pin; these plastic elements are susceptible to a high rate of wear caused by friction, potentially leading to a leakage of fluid from the hydraulic cavity. Finally, the configuration of the linkage housing necessitates that hydraulic lines be connected to two independently rotatable elements, the tubular housing and the linkage housing, thereby exposing these lines to unnecessary stress.
The present invention overcomes the difficulties of previous snubbers through a simple new design.